Negative hydrogen sources for beam currents between one milliampere and one ampere Page: 3 of 12
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Distinction between dissociative attach-
ment and dissociative recombination is that the
dissociative attachment reaction has a signi-
ficant isotope effect up to an electron energy
around 15 eV.
A comparison of these processes suggest
that dissociative attachment and dissociative
recombination are probably the main processes
for the creation of negative ions in a plasma.
In particular dissociative recombination between
low energy electrons and molecular ions (with
a theoretical cross section of 10-17 ca?) is
three orders of magnitude above the values for
dissociative attachment.16 Supplemental cal-
culations are required; experimental verifica-
tion of dissociative recombination is contem-
plated by B. Peart et al.17
The collision processes for the destruc-
tion of negative ions are the reactions be-
tween the negative ion and electron as well as
between the negative ion and the neutral or
charged hydrogen particleCharge Transfer +
H +iH - H + H2
Collisional Detachment
H- + e - H + 2e
H + H - 2H + e
H-+1 - H+H2 +e
Associative Detachment
H + H - H2 + e
Dissociative Attachment
H +H20 -OH +H2(6)
(7)
(8)
(9)
(10)
(11)
(12)The experimental destruction data as a func-
tion of energy are shown in Fig. 2, while in
Table I the maximum destruction cross sections
and corresponding reaction rates are summar-
ized.
Comparison of the production and des-
truction collisions show that the cross sec-
tions for destruction are much larger (three
orders of magnitude) than the cross sections
for creation of such an ion.
An important "destruction" mechanism is
charge exchange between a fast negative ion
and a slow atom, leading to a slow negative
ion and a fast atomH +H - H + H
(13)
It was suggested by Bel'chenko et al.,7,11
that this reaction plays an important role in
the population of H particles in the magne-
tron.1o13
N
bH-+r-N +
J ---- H++
F --.H+2 H H-+H-+2H+e
H +H2-*H+H2+e10-4
--'ci
1
10 I10
W (ev)I
100 1000FIG. 2. Negative hydrogen destruction cross
sections as a function of energy.
3. Charge Exchange Collisions in
Hydrogen and Alkali Vapors
Knowledge of the elementary processes,
which govern the indirect production of nega-
tive ions, is not only required for the under-
standing of charge exchange sources, but they
may well be of interest for plasma sources as
well. Therefore, the energy range of inter-
est considered here, starts from several eV to
the resonant energies. It should be mention-
ed that the experimental data for the low
(< 1 keV) energy range are very limited by
practical limitation of the measuring equip-
ment.
During passage of energetic protons
though gases or vapors the dominant process
in the formation of negative hydrogen ions is
the double charge exchange collisions. The
first charge exchange process is the forma-
tion of neutral atoms by electron capture
H+ + X - H(Is,2s,2p...) + X+ + AE
(14)
in which Xo is the target particle and the
energy defect AE is the difference between
the internal energies of particles before
and after the collision process. The prob-
ability of this charge exchange process to
the ground state is presented by al 0. While
the H(2p) radiative state decays immediately
(1.6 x 0rs sec) to the ground state, emit-
ting a Lyman a photon, the H(2s) metastable
state has a long lifetime (0.14 sec) in the
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Sluyters, T. Negative hydrogen sources for beam currents between one milliampere and one ampere, article, April 30, 1975; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc1016204/m1/3/: accessed May 7, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.